Electrical temperature sensors (Thermister, thermocouple)

1. RESISTANCE THERMOMETER

a) based on fact that the electrical resistance of a metal increases linearly with temperature

b) Eg: include a platinum wire resistor, a battery as a source of electrical potential and an ammeter to measure current that is calibrated to indicate temperature

c) - to increase sensitivity, is incorporated into a Wheatstone bridge

2. THERMISTOR

a) little bead of metal oxide, the resistance of which falls exponentially as the temperature rises

b) can be made very small and is therefore cheaper then the platinum thermometer

c) severe changes in temperature (eg heat sterilization) will alter its calibration

3. THERMOCOUPLE

a) Utilizes Seebeck effect: at the junction of 2 dissimilar metals a small voltage is produced [this type of junction referred to as a thermocouple]. A second junction is needed to form a complete electrical circuit. One of the junctions is kept at a constant temperature or electrically compensated (reference junction) while the measuring junction acts as a temperature probe.

b) The measuring probe can be manufactured as a needle; able to be miniaturised

c) Smaller probes have smaller heat capacities than large probes and therefore a shorter response time